1. Field of the Invention
The present invention relates to a solid electrolyte tube for sodium sulfur cells and a process for finishing the surface thereof, particularly, to a solid electrolyte tube having improved durability and reliability and a surface finishing process for producing the same.
2. Description of the Prior Art
Recently, research and development have been conducted of high temperature type sodium sulfur cells which function at 300.about.350.degree. C. and are excellent, from both functional and economical points of view, in application to electric vehicles or night electric power storage, as a secondary battery.
Hitherto known sodium sulfur cells, as shown in FIG. 3, comprise: a cylindrical anode container 1 accommodating an electroconductive material M for anode, such as carbon mat or the like, impregnated with molten sulfur S i.e. an anode active material; a cathode container 3 containing molten metallic sodium Na and being connected with the top end portion of the anode container 1 interposing an insulator ring 2 made of .alpha.-alumina therebetween; and a solid electrolyte tube 4 made of a polycrystalline .beta."-alumina in the form of cylindrical test tube shaped ceramic closed end tube extending downward from its open top end fixed on the inner peripheral portion of said insulator ring 2, which solid electrolyte tube functions to allow sodium ion Na.sup.+, a cathode active material, to permeate selectively. Further, a long and slender cathode tube 5 extending through the cathode container 3 down to the bottom portion of the solid electrolyte tube 4 penetrates and is supported on the central portion of the upper lid of the cathode container 3.
During discharging, sodium ions permeate the solid electrolyte tube 4 and react with the sulfur S in the anode container 1 to form sodium polysulfide, according to the following reaction. EQU 2Na+XS.fwdarw.Na.sub.2 S.sub.x
Alternatively, during charging, a reaction reverse to the above takes place to produce sodium, Na, and sulfur, S.
The solid electrolyte tube 4 of sodium sulfur cells composed as described above, since the material of the tube to be press-molded is a powder containing polycrystalline .beta."-alumina, is required to be molded by the so-called "rubber-press molding process", wherein the above powder containing polycrystalline .beta."-alumina is charged into the gap between an inner rigid mold 11 and an outer rubber mold 12, as shown in FIG. 4, constituting a rubber-press molding apparatus (an isostatic press) which are then introduced into a high pressure vessel to isostatically press the external peripheral surface of the outer rubber mold 12 at a predetermined pressure P. Thus the pressure acts evenly on and over the whole body of the solid electrolyte tube 4 so that it makes the density uniform throughout the molded body.
The solid electrolyte tube 4 obtained by the above described rubber-press molding process, since the inner surface 4b is high-pressure molded with the rigid inner mold 11, has a rather smooth and even inner surface 4b densified with compressed power particles 6 as shown in the left hand side of FIG. 5. Therefore, during electric discharge, sodium ions uniformly permeate the solid electrolyte tube 4 so that the inner surface 4b presents little problem. However, the outer surface 4a of the solid electrolyte tube 4 is molded with the nonrigid rubber mold 12, so that its surface condition is not always smooth and even, as shown on the right hand side of FIG. 5. With respect to the surface roughness, the outer surface 4a shows a very high arithmetical mean deviation of the profile R.sub.a as well as a large maximum height of the profile R.sub.max, as compared with the inner surface 4b. Consequently, in the use condition of the solid electrolyte tube 4, sodium ion Na.sup.+, sulfur S and sodium polysulfide Na.sub.2 S.sub.x which contact with the surface of the solid electrolyte tube are apt to gather in valley portions 8 rather than on peak portions 7, as shown in FIG. 6. Therefore, electric current concentrates on the valley portions 8 so that the valley portions 8 become liable to deteriorate. Alternatively, when the sodium sulfur cells are heated or chilled, thermal stresses are apt to concentrate at the valley portions 8. Therefore, cracks are liable to initiate at the valley portions 8, so that problems are presented such that the life of the solid electrolyte tube 4 is shortened and the reliability thereof as a battery is lowered.